In order to produce a protein using genetic recombination technology, a host suitable for the expression of the protein is used. Examples of the host used to produce proteins include: animal cells such as CHO cells; insects such as silkworm and insect cells; animals such as chicken or bovine; and microorganisms such as Escherichia coli or yeast. Among these, yeast enables a large-scale high-density culture using an inexpensive medium, and it is able to produce proteins at low costs. Moreover, if a secretory signal peptide or the like is used, it is possible to carry out secretory production of proteins into a culture solution, and thus, it becomes easy to purity the proteins. As proteins produced using the aforementioned host, low-molecular-weight antibodies such as scFv and a Fab-type antibody that are next-generation protein pharmaceutical products have attracted attention. However, when such a low-molecular-weight antibody is allowed to express in yeast used as a host, there is a problem regarding low productivity of the antibody, and there is also a fear that a carbon source may have an influence on the product.
It has been reported that, as a means for solving the aforementioned problems, methanol-assimilating yeast, such as yeast of the genus Komagataella, yeast of the genus Ogataea, and yeast of the genus Candida, is used as a host for avoiding the influence of such a carbon source on the product. Moreover, in order to improve the productivity of proteins, a method for producing a Fab-type antibody, which comprises disposing a nucleotide sequence encoding the Fab-type antibody downstream of a promoter such as methanol oxidase or alcohol oxidase that has an activity several times higher than a common promoter, has been reported (Non Patent Literature 1). However, when a protein having a higher-order structure, such as a Fab-type antibody, is allowed to express using a strong promoter as described above, there is a problem that Fab-type antibodies whose conformations are not correctly folded are accumulated in the endoplasmic reticulum, and stress called “endoplasmic reticulum stress” is given to a cell mass.
As stated above, in order to produce, at low costs, a low-molecular-weight antibody such as a Fab-type antibody using yeast as a host, a method of using a promoter having a higher activity than usual has been known, but this method may cause endoplasmic reticulum stress. Accordingly, such a method of using a promoter having a higher activity than usual cannot be considered to be efficient from the viewpoint of high productivity, and thus, the problem has not yet been solved.